The invention relates to an apparatus and a method for a combined two-dimensional detection of height magnitudes of a workpiece. The apparatus and the method are adapted particularly for measuring tasks in the micro system technology.
For optically measuring height structures with the greatest accuracy even in case of stepped workpieces, the vertically scanning white light interferometry (WLI) has been found to be adapted. The usual interferometric arrangements therefor are the Michelson interferometer, the Linnik interferometer and the Mirau interferometer. In the WLI a white light source, typically a halogen lamp is used for illumination. During measurement, the optical path length difference between the measuring beam and the reference beam is continuously increased or decreased, while at distances of less than 100 nm interference images of the workpiece are reproduced, generally with a two-dimensionally resolving pixel sensor (for example, a CCD or a CMOS array). The optical path length change may be effected by a motion of the workpiece in the direction of the interferometer, a motion of the interferometer toward or away from the workpiece, a motion of the interference object lens or a motion of a reference mirror. This process is designated as “vertical scanning”. The intensity course dependent on the optical path length difference for each camera pixel, the so-called Corellogram, is applied to a further signal evaluation.
In the WLI signal evaluation, a distinction is made between the coherence peak evaluation which yields a relatively coarse estimation of the height location of a measuring point with deviations of partially over 100 nm and the phase evaluation which permits measuring uncertainties in the nanometer or sub-nanometer range. The height measuring range may amount to several millimeters.
Lateral geometric features of workpieces may be determined by means of a digital evaluation of pixel images. For microscopic workpieces the measuring microscope is provided with suitable cameras for reproducing such pixel images and for subsequently evaluating them. An advantage of such a process resides in the high measuring speed which makes also possible, during a corresponding synchronization between the image capture and the arrangement of the workpiece, examinations of the dynamic behavior of corresponding workpieces. All microscopic processes are, however, subject to limitations as concerns the attainable lateral resolution because of the deflection-limited imaging. When using visible light, this generally leads to minimum lateral resolutions of about 0.5 μm.
In case the evaluation of lateral structures base on digitalized light microscopic captures is to be performed by one of the above-noted interferometric arrangements, the interference effects necessarily appearing in the interference microscopy have an adverse effect because of the additional image contrasts conditioned thereon. In the presently known systems at least a replacement of the object lens is required to be able to perform measurements based on white light interferometric and image processing. The exact correlation, however, between the height position determined with the white light interferometry and a light microscopic image captured with limited depth sharpness becomes lost. A further drawback resides in the expenses of the additional object lens and an object lens turret which may be required for an automatic changeover between the two measuring modes.
It is the object of the invention to provide assistance in this connection.